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Autopilots which use GPS to fly an ILS

EuroFlyer wrote:

Only, the GS didn’t appear.

Has it appeared before on a LPV, LNAV+V approach?
If not then the problem is an extra connection is required from the GNS/NAV/COM to drive the KI525A which avionics engineers regularly forget to put in. I have an IFD550 with KI525 and KFC150 and had exactly the same problem on my first LPV approach.
Have a look at the comments from NCYankee below from this thread ‘Autopilot won’t couple on a GNSS Approach’
The most common error on a GNS530W is for the installer to use ILS Energize output P5006 Pin 29 instead of ILS/GPS Approach P5001 Pin 14. The latter is supposed to be wired to the ILS Energize input of the autopilot. If the former is wired to the autopilot, then ILS will work great, but GPS vertical guidance will not. I have seen this dozens of times, usually when the original installation was upgraded from a GNS530 to a GNS530W. Since an IFD550 is pin compatible with the GNS530W, I would guess this is your issue. The reason this happens is that experienced installers don’t use the wiring diagrams and just know (incorrectly) that “ILS Energize Out” from the navigator is hooked to “ILS energize In” on the the autopilot. They are just too experienced to need to RTFM.

EGLK, United Kingdom

Maybe I missed it, but can someone point out to me why a GFC500 requires a GPS position to pass on ILS radio signals to the servos?

Is it because it’s a digital/attitude based A/P and it needs GPS to work it’s magic?

ATPL / IFR Instructor
Europe

I don’t think anyone answered the “why” any more than “because the manual says so” (e.g. this post or this one quote the relevant bits).

tmo
EPKP - Kraków, Poland

I don’t think anybody knows the exact answer, and good luck getting one out of Garmin

They seem to be using some GPS-derived data to improve autopilot performance. Or maybe there was yet another weird certification issue which was most easily solved by introducing GPS data, and they won’t be telling you that for sure. Remember Garmin have their own certification capability, just like Boeing (737 MAX ) – they bought UPSAT for this reason; to get the ODA.

It may not even need GPS position to do this. You can get GPS velocity without a position fix, IIRC. Many years ago I was involved on a project (unrelated to aviation) where we were extracting some data from a GPS which was nothing to do with navigation. We were initially just getting the UTC time (it is amazingly hard to build a box which knows the time, and doesn’t need constant tweaking, without internet access, gps, timecode, or something…). You do have to implement more or less an entire GPS receiver just to get UTC, so you don’t save money, but it means you can be up and running without getting any kind of position fix, and you need just 1 satellite.

Administrator
Shoreham EGKA, United Kingdom

It’s the switch of AP mode after the loss of GPS signal that prevents GFC500 from keep tracking ILS

GPS loss or not, you just have to watch it (which is healthy when flying on AP anyway) if it disconnects you press the APP again, nothing really different from flipping NAV1/2 or intermittent GS/NAV flags…

ESSEX, United Kingdom

Peter wrote:

I don’t think anybody knows the exact answer, and good luck getting one out of Garmin

I guess the GFC500 uses the GPS to get the actual track of the aircraft. That will make the A/P control loop much simpler compared to only using heading and course deviation. The wind usually shifts a bit going down the ILS. Knowing the actual track, the A/P can compensate immediately. Without it, a course deviation would have to happen first.

Also by knowing the actual track, the A/P would be much less dependent on the flight characteristics of various aircraft types.

ESKC (Uppsala/Sundbro), Sweden

It’s the switch of AP mode after the loss of GPS signal that prevents GFC500 from keep tracking ILS

You may not be using GPS nav at all. You may be in HDG mode, to intercept the LOC; that is actually the most usual way by far in Europe.

I guess the GFC500 uses the GPS to get the actual track of the aircraft.

Sure, but then why the GFC600/700 don’t have this limitation? It would imply the slightly bizzare situation where you pay more €€€€€ and you get a worse autopilot

Administrator
Shoreham EGKA, United Kingdom

A GFC600 is way more expensive than a GFC500 – $20.000 vs $7.000 as a starting price. They are also marketed differently, the GFC500 is a “cost-effective and full featured retrofit autopilot and flight director solution for select GA light single-engine, fixed-wing aircraft”, whereas the GFC600 is a “full featured, retrofit flight control solution for high-performance single- and twin-engine piston aircraft as well as turboprops and jets”.

tmo
EPKP - Kraków, Poland

It is amazingly hard to build a box which knows the time, and doesn’t need constant tweaking, without internet access, gps, timecode, or something….

DFC77

LDZA LDVA, Croatia

They (Garmin) use GPS altitude information to be more precise on the glideslope.

Safe landings !
EDLN, Germany
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